Examples of a hub bearing for driven wheel as described above are provided in Patent Literature 1, Patent Literature 2 (see FIGS. 11 and 17), and Patent Literature 3 (see FIGS. 4, 8, 12, and 15). In all of these examples, a seal ring is attached between an outer race member and an inner race member on the hub flange side (wheel mounting side). A protective cap is attached to an opening portion of the outer race member on the vehicle body side. The protective cap is formed of a cylindrical part that is fitted to an inner diameter surface of the outer race member and a lid part configured to connect to block an opening end of the cylindrical part on the vehicle body side. The protective cap covers the entire bearing space including the end portion of the inner race member on the vehicle body side. The protective cap functions to prevent entry of soil water from the vehicle body side to the bearing space or external leakage of a lubricant. In the invention of Patent Literature 1, an annular concave part exerting a labyrinth function is formed on the protective cap to prevent the sealing property of the cap from being decreased because of a gap between the outer race member and the protective cap resulting from a temperature difference between the two during operation of the bearing. In each of the hub bearings for driven wheel described in Patent Literatures 2 and 3, since an internal pressure of the bearing space fluctuates during operation of the bearing, the protective cap is provided with a means for adjusting the internal pressure.
According to other configurations described in Patent Literatures 2 and 3 and an example described in Patent Literature 4, the seal ring for sealing a bearing space between the outer race member and the inner race member is provided with an adjustment means for adjusting an internal pressure of the bearing space. In addition, according to a technique disclosed in Patent Literature 5, a groove, a through hole, a notch, or the like is formed in a cap attached to a bearing hole that supports a shaft of a throttle valve via a bearing. During pressing of the cap into the bearing hole, therefore, the cap for the bearing hole can be prevented from causing an increase in internal pressure of the sealed space.